Ok Entry Level Amp, but not the best.
Ahh the Fender Mustang. One of the all time most popular practice amps, and not without good reason. I have owned one of these for almost four years now, and it has served me very well.
Lets talk non sound related things first: even the biggest of amp snobs cannot deny that this thing is excellently screwed together. The whole amp feels very solid, the knobs turn smooth and silent, the jacks are high quality, and nothing feels cheap or poorly made. The amp is very handsome with the traditional Fender look and feel of old blackface amps. It has taken more than its fair share of bumps in the time I've owned it and it has held together like a total champ.
Now on to sound, in my opinion this amp is definitely passable but very hit or miss, considering the price however it's quite good. Before I get started though, my one biggest complaint about this amp: PLEASE INCLUDE A MID CONTROL!! Seriously what is the deal with no mid control knobs? Anyway, in my opinion this amp is vastly better when clean than when distorted, and the clean tones can actually be excellent.
My favorite setting of all of them is the green channel '59 Bassman emulation, it is phenomenally accurate to a real Fender combo, and is perfectly balanced and natural sounding (it also gets along with pedals very nicely) I run a pretty extensive pedalboard so I disable all of the inbuilt effects almost always and I've edited the '59 Bassman setting to not use them. The other Fender recreations are also good, the '65 Twin Reverb is good for Jazz sounds, and when paired with some of my pedals can sound quite beautiful for ambient clean sounds.
Speaking of disabling effects and such, I should mention that I think Fender tried a bit too hard with some of these sounds, the effects are there but there is little to no control over them other than on or off so it's all or nothing and I find that these effects are often overpowering and fake sounding, some are ok but that's about it. The sheer number of amp models and sounds is impressive, but I find that I use very few of them and most just wind up sitting there. The '59 Bassman, British 60's, British 80's, and American 90's settings on the green and amber channels are good, but I find the others really to be rather useless. I am the kind of person who uses all of the settings available to me on most things but still I think that a lot of these sounds were undercooked and they probably should consolidate to a smaller number of really well done sounds if they want to shed the stigma of practice amps being cheap sounding.
Back to the amp sound, while I mentioned that this thing's forte is definitely clean sounds, it is capable of classic drive as well (a slightly pushed Bassman on the green channel with a Tube Screamer in front of it produces a gorgeous vintage drive sound), but really overdrive sounds out of the speaker are not the best with the exception of a few like the setting I just mentioned. The reason for this isn't to do with the way the circuitry is put together, more with the fact that Fender opted for an 8" speaker instead of a full 12" that is standard on most amps. This cuts the low end and it needs to be artificially added back in to compensate which often hurts this amp's driven sounds. In terms of metal this thing often will just fall flat due to this speaker issue, but when running direct into a recording interface with a cabinet impulse it can do much better.
Speaking of metal I should mention that I did figure out a good metal sound with the speaker: go to green channel American 90's, set the gain at 4-4.5, the volume at 6.5-7, the bass on 6 and the treble on 5 (adjust master to taste) and with a vintage style Les Paul in the bridge position it will rip (a low gain Tube Screamer is also a good addition to this signal chain for a bit more oomph if need be).
This leads me to gain itself: on the higher gain settings the default is WAY oversaturated, and I have seen several new guitarists who do not know much about tone yet be led into bad habits by this over the top sound that in a mix is awful and causes them to lose all the dynamics in their playing.
At the end of the day though this amp is $, it's not a multi-thousand dollar Mesa Boogie so you can't expect to fairly compare them, and while I have been fairly critical in this review I should say it is not because I insist on fancy expensive amps, the Fender Champion 40 is a vastly better beginner option, it's not much more expensive than the mustang, but has twice the power, a full 12 inch speaker, an open-back cabinet (allows for easy speaker change), adjustable level effects, a fully independent clean channel, and while there are less presets they're all useful and can produce a wide variety of tones with a lot more character and style than the Mustang.
While the Mustang does have the USB out going for it which is nice, both these amps have direct headphone outs which can be run into an interface for a direct silent recording setup.
I think for the beginning guitarist who does not know for sure the tone they're after, the Champion 40 would be a better option overall but you honestly can't go wrong with either one of these amps.